Fujitsu develops nanotube-based heatsinks

Fujitsu has developed carbon nanotube-based heat sinks for use in high-frequency power amplifiers targeted at mobile communications infrastructure, scientists from the company revealed at the International Electron Devices Meeting in Washington, D.C.

The researchers said that the use of carbon nanotubes as heat sinks for high-frequency high power amplifiers achieves heat dissipation and high amplification simultaneously.

They also claim that the technology represents a major advance in developing practical applications that take advantage of the thermal conductivity of carbon nanotubes.

Fujitsu said the work, part of a project in collaboration with the Japan Fine Ceramics Centre, is likely to see commercialization in three years' time. The company also said that the focus now is to refine the site density of carbon nanotubes in bumps to enable further improvements in heat dissipation prior to developing the high-frequency, high power flip-chip amplifiers using carbon nanotube bumps.

The researchers say the motivation for the research was that at higher frequencies, amplifiers using the traditional "face-up" structure suffer from reduced amplification. This is due to inductance from the metal wire through which the electrical current flows from the electrode of the transistor chip to the electrode of the package.

Flipping over the chip and connecting the chip electrode and the package electrode with short metallic bumps made from gold or other metals, is a solution to the problem. However, for use in high power amplifiers, conventional metallic bumps have proven inadequate in dissipating the high levels of heat generated by high-power transistors.

Key advances claimed by the researchers include novel techniques to grow carbon nanotubes. They used an iron catalyst coating to grow carbon nanotubes to a vertical length of at least 15µm on the wafer substrate. Usually, bumps for flip-chips are required to have a length of at least 10µm.

They have also developed new methods to connect the carbon nanotube bump to the flip-chip. Fujitsu said the most recent research has yielded flip-chip structures with heat dissipation equivalent to conventional face-up structures, ground inductance that is reduced by more than half, and an increase in amplification of at least 2dB at frequencies of 5GHz or greater.